A punching apparatus for web material, as is to be found in EP 0 167 018, has a punching tool which, if necessary, prior to each punching operation is aligned with the respective region which is to be punched out of the web material, that region being provided for example with printing thereon. The design configuration of that apparatus is such that the punching tool is arranged on a stationary base, with the interposition of a plate which is displaceable transversely relative to the longitudinal direction of the web material and also pivotable. The punching tool in that apparatus comprises two tool portions, constituting an upper tool portion and a lower tool portion. The upper tool portion is suitably carried by the lower tool portion, for which purpose the latter is provided with columns on which the upper tool portion is guided. Disposed around the columns are compression springs which hold the upper tool portion at a spacing from the lower tool portion. Arranged above the upper tool portion is a piston-cylinder unit which, for the purposes of effecting the punching operation, displaces the upper tool portion towards the lower tool portion, against the force of the compression springs. After the punching operation has been carried out the piston of the piston-cylinder unit is moved back upwardly into its starting position, in which case the upper tool portion is also moved back upwardly into its starting position in which it is at a spacing from the lower tool portion.
As the punching tool assembly consisting of the upper and lower tool portions is relatively heavy and may be for example more than 100 kg in weight, particular precautions are usually required to displace and pivot the transversely displaceable and pivotable plate which bears the weight of the punching tool assembly, with the aim of reducing the frictional forces between the transversely displaceable and pivotable plate and the stationary plate on which it is carried, so as to permit precise adjustment and setting of the movable plate in order to achieve satisfactory accuracy in the punching operation on the web material. In addition the forces required for adjusting the movable plate should not exceed a certain level, having regard to the power of the control motors required for that purpose.
In the punching apparatus in above-mentioned EP 0 167 018, to achieve the above aim the upper movable plate which thus performs both a transverse movement and also a pivotal movement is provided with means which permit the formation of an air cushion between the upper movable plate and the lower stationary plate, for adjustment of the punching tool assembly. The air cushion slightly lifts the upper plate which carries the punching tool assembly, from the lower stationary plate, so that the adjusting movements of the upper plate, which then take place, involve a markedly reduced amount of friction between the upper movable plate and the lower stationary plate which supports the upper plate. That arrangement however suffers inter alia from the disadvantage that the air cushion which thus supports the upper plate with the punching tool assembly in the adjustment procedure cannot provide any guarantee that the upper plate is always disposed parallel to the stationary plate therebeneath. A departure from a condition of parallelism between the two plates can occur for example by the upper plate involving an irregular distribution of weight over its area, so that the unequal weight loading causes the upper plate to tilt slightly relative to the lower plate. That departure from a parallel condition as between the upper and lower plates also affects the punching tool assembly which is carried by the upper plate, and can result in the final position of the upper plate with punching tool assembly carried thereon not precisely corresponding to the desired or reference position after the upper plate is lowered on to the lower plate again, due to elimination of the air cushion therebetween. The upper and lower plates would then no longer be accurately positioned relative to each other. Although the deviations which may occur in such a situation may admittedly be very slight, it is to be borne in mind that the requirements in terms of the level of accuracy with which the punching tool assembly is to be aligned with respect to the region to be punched out of the web material may be of an order of magnitude of less than 10 .mu.m. A further difficulty with that punching apparatus is that the stationary plate must be relieved of the load of the weight of the entire punching tool assembly as the lower tool portion also supports the upper tool portion, by way of the columns and compression springs.